Behaviour of RC Structural Elements With Laced Reinforcement

Available online:

https://edupediapublications.org/journals/index.php/IJR/

Behaviour of Rc Structural Elements With Laced

Reinforcement

Tamma Ashok Reddy, & dr.D.Venkateswarlu,

1_{Pg Scholar,Godavari Institute of Engineering & Technology, Rajahmundry}

2_{Professor& HOD, Department of Civil Engineering, Godavari Institute of Engineering &}

Technology, Rajahmundry

ABSTRACT:

Reinforced concrete due to its special

properties like high compressive strength,

fire resistant, durability etc., is considered

as most widely accepted construction

material in construction industry, but by

placing reinforcement in some conventional

forms, the properties of reinforced concrete

elements showing significant poor results

compared to some other alternative

techniques of placing reinforcement. By

using same percentage of steel and by

altering pattern or position of

reinforcement, the behaviour of reinforced

concrete elements differ which will give

significant upper bound results. Among the

alternative techniques, Laced Reinforced

Concrete (LRC) is most adaptable method

of placing reinforcement and can be

implemented in the structural RC elements

like slabs, beams and floors. By introducing

these inclined laced bars the concrete

confinement area increases and this

maintains structural integrity. By providing

these inclined bars perpendicular to the

direction of propagation of crack,

formation of cracks are arrested. RC

element incorporated with LRC can resist

impact loads, earthquake loads and blast

loads. This dissertation work deals with

two types of structural elements and

describes the behaviour of parameters like

ultimate load carrying capacity,

deflections, and crack width by adopting

the RC elements with LRC. In first series

the investigations conducted on four beam

elements of 1.5m length, 0.15m x 0.3m

cross section area.. The deflections and

crack width are taken at each and every

step load by using dial gauge having least

count of 0.01mm and hand hold microscope

having least count of 0.02mm. The test

results are tabulated and the graphs are

drawn between load vs. deflection and

moment vs. crack width and are compared

with their companion specimens. The aim,

Available online:

https://edupediapublications.org/journals/index.php/IJR/

improved ductility, high member strength,

reduced crack width and deflection.

INTRODUCTION Blast loads due to terrorist attacks and chemical and nuclear

outbreaks are very often taking place all

over the world causing the structures to

collapse. For the structures imperiled to

suddenly applied loads and imposed loads

structural integrity and ductility are

fundamentally required. By implementing

the LRC to the structural elements, ductility

and structural integrity and better concrete

confinement of the element enriches.

Design of blast resistant structures with

conventional methods are highly

uneconomical, therefore to produce cost

effective solution by considering safe and

serviceability taking into account to resist

blast and impact loads achieved, by

adopting LRC. In Reinforced concrete

beams with conventional stirrups, shear

span to depth ratio is less than 2.5, ductile

failure is not promising due to the effect of

diagonal cracking, this can be achieved by

implementing the Laced reinforcement, and

apart from this it also increases the

ductility, tensile strength and ultimate

strength of concrete. Spall of concrete takes

place of any element with conventional

concrete at failure load, but greater

confinement of concrete can be obtained

with laced reinforcement which maintains

high structural integrity so the spall of

concrete is restricted and the specimen will

fails at the ultimate load with small cracks.

Laced reinforcement as a Lattice girders

reinforcement are three dimensional

metallic structures consist of an upper

chord, two lower chords and one

continuous diagonal bars which are tied to

the chords at a specified nodes or specified

angles. Laced reinforcement as a lattice

girder reinforcement gives additional

structural integrity by its truss action so that

members can with stand higher ultimate

loads without spall of concrete at failure. In

precast industries this lattice girder

reinforcement widely used in amalgamation

with Hybrid concrete construction (HCC),

which is insitu and precast concrete

combination to obtain the benefits from

these two method constructions. In precast

industry the lattice girder reinforced slabs

are casted in two stages, in first stage

nearly 50-60mm thick concrete is casted in

industry (casting yard) and the remaining

thickness of slabs are casted after erecting

at the site of construction. The projections

from the lattice girder slab provides

additional stiffness while erecting and also

maintains composite action between first

Available online:

https://edupediapublications.org/journals/index.php/IJR/

2.LITERATURE REVIEW:

Anandavalli et al., has performed experimental studies on “Behaviour of a

blast loaded laced reinforced concrete

structure”. From these studies, the authors

concluded that, when the structures that are

designed for reverse loads such as blast and

earthquake loads LRC is economical, in

comparison with R.C.C. At a deflection

corresponding to 20 support rotation the

concrete in compression crushes. The

structural elements without shear

reinforcement loses its structural integrity

at 20 support rotation due to lack of

confinement of concrete. The structural

members with conventional two legged

closed stirrups loses its structural integrity

at 40 support rotation and the structural

members arranged with laced

reinforcement due to its truss action, the

reinforcement in the members will resist up

to 120 _{until tension failure of reinforcement}

take place. Here the authors have

conducted tests with lacing angles between

450 _{to 60}0_{. In this study two storage tanks}

were constructed, first tank is donor and the

second one is acceptor tank which is laced

reinforced structure. In donor tank

explosives are placed and the separation

distance between two tanks are reduced

with the aim that explosions caused in the

donor structure would not damage the

acceptor structure. The test is monitored by

placing pellets on beams and roof of the

acceptor tank for measuring strains and

deflections. From the experimental

investigations the cracks pattern of the

acceptor slab followed the expected yield

line pattern. From the experimental results

the deflections for roof slabs are less and

the acceptor structure is serviceable after

explosion. The reduction in existing

provision of separation distance from

2.4W1/3 to 0.7W1/3.

Srinivasa Rao et al., conducted the experimental investigations on “seismic

behavior of laced reinforced concrete

beams”. This paper mainly focuses on the

ductile property of laced reinforced

concrete (LRC). The lacing reinforcement

is placed in plane of principal bending and

these bars are kept in position with the help

of transverse bars. When the shear span to

the depth ratio is less than 2.5, ductile

failure of the beam will not happen with the

use of conventional stirrup reinforcement.

By altering pattern of shear reinforcement

with inclined bars improved ductility can

be achieved. After conducting tests on 20

LRC specimens, keeping flexural

reinforcement same and changing different

Available online:

https://edupediapublications.org/journals/index.php/IJR/

welded, inclined tied, single leg lacing and

rectangular lacing with normal and fiber

reinforced concrete, under static and cyclic

loading conditions it was concluded that

inclined lacing with or without fiber

reinforcement given better results

compared to remaining specimens. By

providing laced reinforcement ductile

failure achieved when the specimens are

subjected to cyclic loading. As there is no

unified procedure for finding ductility, here

ductility evaluation is done as per park and

sheik et al which suggests that 75% of peak

lateral load gives the yield deformation and

ultimate deformation corresponding to 85%

of peak lateral load. Conventional

reinforcement in combination with laced

shear reinforcement gives additional

confinement at plastic hinge locations. As

the cost of fabrication increase by using

laced reinforcement it can be reduced by

adopting tack welding to the lacing.

Stanleyc. Woodson et al led the

experimental study on “Stirrup

Requirements for Blast – Resistance

Slabs”. The study aimed to evaluate

effective placing or varying parameters has

significant effect on strength and

deformation characteristics on one way

slabs. Here by considering three varieties of

stirrup configurations, ten one way slab

elements were investigated to find the shear

capacity by inducing uniform pressure. The

study focuses on three parameters like

configuration of stirrups, stirrup spacing

and stirrups interactions. Here the stirrup

configuration are varied with U- shaped,

double leg and single leg stirrups provided

with hook angle of 1350. After obtaining

the results, the authors conclude that closed

stirrups provided to the specimen has

shown more deflections. Here the author

finally concludes that, closely spacing of

single legged stirrups is suitable method to

resist against blast loads and it is cost

effective solution than laced reinforcement.

Here dobbs and dede state that members

arranged with laced reinforcement due to

its truss action, the reinforcement in the

members will resist up to 120 until tension

failure of reinforcement take place.

3. PROCEDURE OF TESTING SPECIMENS

In this methodology, the equipment used

was loading frame provided with hydraulic

jack which transfers the static imposed

loads to the specimen.

1. The marking are prepared on the

test specimens at a distance of 100mm from

both faces, for locating the seating position

Available online:

https://edupediapublications.org/journals/index.php/IJR/

2. The markings are also made for

location of point of application of load and

point of placement of dial gauge for

recording deflections.

3. Then the specimen is placed on

the supports, so that markings made on the

specimen should coincide with supports. In

this dissertation work the supports

considered are simply supports made with

heavy steel girders.

4. After making all the adjustments,

the machine is started and required settings

are made in the input window. The loading

frame used in this dissertation work had a

capacity of 100T but here it is set to 50T.

5. Then the soleden of the hydraulic

jack is moved downward until a gap of

4-5mm is maintained between soleden and

specimen and in the input window set the

step load for 10kN.

6. After load transfer started

deflections and maximum the crack width

are noted at every subsequent step load.

7. After recording the deflections

and the crack width at previous step load,

we continue for next step load.

8. When the specimen had failed due

to the applied load it has reached its

ultimate load, then the load is released.

9. Then the failure pattern is

observed and it is photographed.

10. The input window is shown in

below figure. It consists of

a) Sample no.

b) Maximum load in Tons.

c) Step load in Tons

d) Raising ramp in seconds

e) Step load holding time in seconds.

f)Current load in Tons.

g) Current step

h) Total steps

Step by step Procedure of loading frame operation:

1. First switch on the mains.

2. Secondly, switch on the motor.

3. On the input window set the

sample no, maximum load, step load,

raising ramp in seconds, step load holding

Available online:

https://edupediapublications.org/journals/index.php/IJR/

4. The movement of soleden is done

by changing the mode to manual mode,

then the soleden of the hydraulic jack is

moved downward until a gap of 4-5mm is

maintained between soleden and specimen.

5. After the sole den reaches the

required distance the solden should be kept

in neutral mode

6. Before starting system the mode

is changed from manual to auto. Then

system have to be started.

7. An icon will blink after

completing every step load on the input

screen, which specifies us, to continue for

next step load.

8. By clicking on the ‘yes’ the next

step load will be applied on the specimen.

9. If the specimen had failed due to

the applied load it has reached to its

ultimate load and no more load transferring

takes place.

10. After specimen had failed, the

system should be stopped.

11. The mode is changed from auto to

manual and the solden is raised upward so

that load is released and keep the solden in

neutral.

12. Then stop the motor

COMPARISION AND DISCUSSION OF TEST RESULTS

The results obtained from the tests are

tabulated at every step load and presented in

chapter 4. In first series the comparison of

test results between the specimens adopted

with two legged vertical shear

reinforcement and laced reinforced

reinforcement of 4mm and 6mm diameter.

In the second series comparison made

between conventional bottom mesh

reinforcement (S1) and laced reinforcement

as lattice girder reinforcement with full (S2)

and two stage(S3) casted slabs.

Table 5.1 Principal test results of beams

Name of Specimen

Design load(kN)

Ultimate load(kN)

Deflection(mm) at Crack width(mm) at

Design load

Ultimate load

Design load

Ultimat e load

B1 100 110 3.99 4.55 0.36 0.4

B2 100 170 2.59 5.70 0.22 0.80

Available online:

https://edupediapublications.org/journals/index.php/IJR/

B4 100 200 2.19 6.10 0.06 1.12

Table 5.2 Principal test results of slabs

Name of Specimen

Design load (kN)

Ultimate load (kN)

Deflection(mm) at Crack width(mm) at

Design load

Ultimate load

Design load

Ultima te load

S1 75 140 4.075 19.4 ---- 3.4

S2 75 160 1.79 14 ---- 3.2

Available online:

https://edupediapublications.org/journals/index.php/IJR/

From all the comparison tables, graphs and failure patterns, the specimens:

Beam Specimen B1: For specimen B1, thedeflections atdesign load (100kN) and

ultimate load (110kN) is 3.99mm and

4.55mm and crack width at design load

and ultimate load are 0.36mm and 0.4mm.

In this the specimen the first crack

observed at 80kN with increment of loads

shear cracks are formed, and these cracks Fig. 5.4 Modes of cracks for B1 at failure Fig. 5.5 Modes of cracks for B2 at failure

Fig. 5.6 Modes of cracks for B3 at failure _{Fig. 5.7 Modes of cracks for B4 at failure}

Fig. 5.8 Modes of cracks for S1 at failure Fig. 5.9 Modes of cracks for S2 at failure

Available online:

https://edupediapublications.org/journals/index.php/IJR/

are propagated to full depth and failed in

shear.

Beam Specimen B2: For specimen B1, thedeflections atdesign load (100kN) and

ultimate load (170kN) is 2.59mm and

5.7mm and crack width atdesign load and

ultimate load are 0.22mm and 0.8mm. In

this the specimen, the first crack is flexural

crack observed at 50kN with increment of

loads shear cracks are also formed, and

these cracks are propagated to full depth

and failed in flexure with partial shear.

Specimen B3: The specimen B3 is provided with laced reinforcement of 4mm

in the replacement of conventional

reinforcement it has failed at an ultimate

load of 160kN. In this the first crack

formed at 80kN. Thedeflections atdesign

load (100kN) and ultimate load (160kN) is

2.95mm and 6.10mm and crack width at

design load and ultimate load are 0.2mm

and 1.24mm. The beam B3 has shown

nearly similar results compared to B2,

though the percentage of shear

reinforcement was 58% of B2 only.

Specimen B4: The specimen B4 is provided with laced reinforcement of 6mm

in the replacement of conventional

reinforcement it has failed at an ultimate

load of 200kN. In this the first crack

formed at 100kN. The deflections at

design load (100kN) and ultimate load

(160kN) is 2.19mm and 6.10mm and crack

width at design load and ultimate load are

0.06mm and 1.12mm. The beam B4 has

shown greater results compared to B2, and

the percentage of shear reinforcement was

same for B2 and B4. The type of failure is

shear failure. Due to more concrete

confinement and truss action the member

attains more structural integrity and

reached greater ultimate loads.

Specimen S1: For specimen S1, the deflections at design load (75kN) and

ultimate load (140kN) are 4.075mm and

19.5mm. At design load there is no crack

formed and at ultimate load a crack of

3.4mm is obseved. In this the specimen the

first crack observed at 100kN. It has

shown limited ductility when compared to

the S2 and S3. Initially flexural cracks

appeared but it had failed in punching

shear.

Specimen S2: The full casted slab with lattice girder reinforcement the deflections

at design load (75kN) and ultimate load

(160kN) are 1.79mm and 14mm. At

design load there is no crack formed and at

ultimate load a crack of 3.4mm is

Available online:

https://edupediapublications.org/journals/index.php/IJR/

crack observed at 110kN. It has shown

high load carrying capacity and achieved

higher ductility when compared to the S2

and S3. Initially flexural cracks appeared

but it had failed in punching shear.

Specimen S3: The two stage casted slab with lattice girder reinforcement the

deflections at design load (75kN) and

ultimate load (150kN) are 1.755mm and

14.8mm. At design load there is no crack

formed and at ultimate load a crack of

2.9mm is observed. In this the specimen

the first crack observed at 100kN. It has

shown high load carrying capacity and

achieved higher ductility when compared

to the S2 and S3. Initially flexural cracks

appeared but it had failed in punching

shear. It has achieved high ductility when

compared to the S1. The behavior of stage

wise casted slab is merely coinciding with

the behavior of full casted slab. Initially

flexural cracks appeared but it had failed

in punching shear.

COMPARISION OF RESULTS FOR BEAMS:

B1 has been provided without

conventional shear reinforcement. It has

been failed at 110kN if the same beam is

provided with conventional shear

reinforcement it has reached its ultimate

load at 170kN shows the importance of the

shear reinforcement in beams. Here we are

choosing another pattern of shear

reinforcement which is provided in the

replacement of conventional shear and it is

provided in the longitudinal direction

connecting to top and bottom main bars

and is inclined 50 degrees throughout the

length of the beam. The same laced

reinforced pattern is used for B3 & B4

with 4 mm and 6mm diameter

respectively.

By using this kind of laced reinforcement

B3 has failed at 160kN which is 10kN less

by conventional shear but here amount of

shear steel is used was 58% of B2.

While B4 has reached its ultimate load at

200kN which was 30kN more than B2 at

same time the amount of steel used was

only 3% more than B2.by altering the

pattern of shear reinforcement the load

carrying capacity was increased to 17.5%

of B2.

DEFLECTION:

For B1 at design load the deflection was

3.99, by using conventional shear

reinforcement in B2 it has deflection at

design load was 2.59 and for B3 it was

Available online:

https://edupediapublications.org/journals/index.php/IJR/

than B2 and for B4 the deflection was 2.19

which is 15.5% less than B2, at failure

load B2 the deflection was 5.7 mm at that

load B4 has deflection of 4.05 mm which

is 30% less compared to B2.

CRACK WIDTH:

For B1 without shear reinforcement the

first crack width started at 80kN at that

load B2 has a crack width of 0.14 mm, for

B3 it has 0.09 and for B4 there is no crack

formed at that load. At design load the

crack width for B1 was 0.36, for B2 0.22,

for B3 0.2 which is 10% less by B2 and

for B4 it is 0.06 which is a first crack and

it is 72% less by B2.

Discussion of test results for slabs specimens:

 T

he ultimate load carrying capacity of S2, S3 had been increased to 15% and 7.5% when compared to S1 respectively.

 A

t design load, the deflection of S2 , S3 was reduced by 56% and 57% of S1 respectively

 A

t ultimate load of S1, the deflection of S1 is 19.4 at that load the deflection of S2 and S3 is 5.92 and 6.8 which is 69.7% 65.12 less by S1

 A

t ultimate load S1, the crack width for S1 was 3.4mm at that load crack width for S2

and S3 was 0.5mm and 0.86 which is which is 85.3% and 74.7 % less by S1.

 A

t each load increment, deflections of S2 and S3 has nearly 0.4 to 0.7 times of S1.

 A

t each load increment, crack widths of S2 and S3 has nearly 0.1 to 0.4 times of S1.

CONCLUSIONS

1. The specimens with laced

reinforced beams have failed at greater

ultimate load which is 10 to 20% more

than their companion specimens.

2. In case of lattice girder slabs

ultimate load carrying capacity is 10 to

15% more to their companion specimens

3. At design load and ultimate loads,

the deflections of laced reinforced beams

reduced by 10 to 15% and 30% to their

companion specimens respectively.

4. At design load and ultimate loads,

the deflections of lattice girder reinforced

slabs are reduced by 55 to 60% and 65 to

70% to their companion specimens

respectively.

5. At design load and ultimate loads,

the crack widths of laced reinforced beams

reduced by 36% and 72% to their

Available online:

https://edupediapublications.org/journals/index.php/IJR/

6. At ultimate loads, the crack

widths of lattice girder reinforced slabs are

reduced by 75 to 80% to their companion

specimens.

7. The specimens of both series with

laced reinforcements showed more

ductility than the companion specimens.

8. The members achieved greater

structural integrity and spall of concrete

restricted due to truss action with the

provision of laced reinforcement

9. Laced reinforcement due to its

truss action which gives more structural

integrity, so that the member can

withstand higher ultimate loads without

spall of concrete at failure.

10. From the experimental

investigation laced reinforcement with

lacing angle of 500 for beams and slab

specimens with respect to principal

reinforcement achieved the improved

ductility, lesser deflections and Control in

crack width than their companion

specimens, so these are preferable lacing

angles to RC elements.

11. Lattice girder reinforcement with

2 stage casting gives a closer test results

when compared to full casted lattice girder

slabs.

12. From load vs. deflections and

Moment vs. crack widths graphs shows

close resemblance between full casted and

casted in two stages slab elements, by this

we can conclude that lattice girder

reinforcement can be used in precast

industry in order to maximize the benefits

of both precast and cast insitu methods.

REFERENCES

[1] Anandavalli, N., Nagesh R. Iyer, Lakshmanan, N., K., Rajasankar, Amar

Prakash, Chitra Rajagopal and

Ramanjaneyulu, J., “Behaviorof Blast

Loaded Laced

ReinforcedConcreteStructure”. DSJ,

volume .62, NO. 5, Septe 2012, pp. 284 – 289.

[2] Akshaya, S. G., R., Vishnupriya. B., Anantha Krishnan, G., Sanjeevi. R., Arunprasad, Manikandan. “Experimental studies on lacedsteel concretecomposite elements extremeloading condition”. ISOR journal of Mechanical andcivil engineering, e-ISSN: 2278-1684, pp. 54 – 61.

[3] Srinivasa Rao, Lakshmanan, N., P., Sarma, B.S., and Stangenberg, F., “Seismic Behavior of Laced Reinforced

Concrete Beams”.

EleventhWorldConference on Earthquake Engineering, 1996, pp. No. 1740.

Available online:

https://edupediapublications.org/journals/index.php/IJR/

Beams under Monotonic Loading”.

Advancein StructuralEngineering, 2015, pp. 355 – 367.

[5] Kinger, S., and Stanleyc,

Woodson., “Stirrup Requirements for BlastResistance Slabs”. J.struct.Eng.,1988

[6] Tapan, M., StructuralResponse of

ReinforcedConcrete WideBeams

Reinforced With Lattice Girders”. IJST, Vol. 38, No. C2, pp 337-344

[8] Figueiredo filho, J. R., Shiramizu,A. K. H., “Design, manufacture and construction of buildings with precast lattice-reinforced concrete slabs” ISSN , Volume 4, Number 1, p.p 123 - 146

[9] IS 456 2000, “Plane and

Reinforced Concrete – Code of

Practice”, Bureau of Indian Standards, 2000.

[10] SP34 : 1987, “Handbook on

ReinforcedConcrete and Detailing”,